Citizen Action Monitor

Mathematician joins in on the human population controversy with some cold hard facts

If everyone lived like Americans, Earth could only support about 1.5 of the current population of 7.5 billion

No 2332 Posted by fw, July 11, 2018

Andrew Hwang

“Humans are the most populous large mammal on Earth today, and probably in all of geological history. This World Population Day, humans number in the vicinity of 7.5 to 7.6 billion individuals. Can the Earth support this many people indefinitely? What will happen if we do nothing to manage future population growth and total resource use? These complex questions are ecological, political, ethical – and urgent. Simple mathematics shows why, shedding light on our species’ ecological footprint.”Andrew D. Hwang, The Conversation

Dr. Hwang is an Associate Professor of Mathematics, College of the Holy Cross in Worcester, Mass.

Following his introduction, Dr. Hwang continues his article with what it means to say human population growth is ‘exponential’ – the term refers to the length of time it takes for our population to double. For a human population, doubling time is not constant – in the year 1800 the population had doubled in 300 years; 127 years later in  1927 it had doubled again; in just 47 years in 1974 it doubled; in 49 years in 2023 it doubled; and the population is expected to level off at 10 to 12 billion in another 79 years in 2100. Earth’s carrying capacity will cut short population growth when premature death by starvation and disease balances the birth rate.

Here are a few more key facts from Dr. Hwang’s article –

  • If every person on Earth were to live like Americans, our planet could only support about 1.5 billion of the current 7.5 billion people
  • If 7.5 billion people consumed water at American levels, world usage would be about 11% of the Earth’s total water supply in just one year
  • Global consumption is rapidly drawing down non-renewable resources including fertile topsoil, drinkable water, forests, fisheries and petroleum.
  • Population stays constant when couples have about 2 children who survive to reproductive age; in some parts of the developing world today, couples average 3 to 6 children

Humans are living far beyond Earth’s sustainable means, argues Hwang. Encouraging smaller families, is our most humane hope.

Below is a slightly abridged repost of Hwang’s article with my added subheadings and text highlighting. To read his piece on The Conversation’s website, click on the following linked title.

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7.5 billion and counting: How many humans can the Earth support? by Andrew D. Hwang, The Conversation, July 9, 2018

Humans are the most populous large mammal on Earth today

Humans are the most populous large mammal on Earth today, and probably in all of geological history. This World Population Day, humans number in the vicinity of 7.5 to 7.6 billion individuals.

Shedding light on our species’ ecological footprint

Can the Earth support this many people indefinitely? What will happen if we do nothing to manage future population growth and total resource use? These complex questions are ecological, political, ethical – and urgent. Simple mathematics shows why, shedding light on our species’ ecological footprint.

The mathematics of population growth

“Exponential growth” in this case, refers to the time it takes for a human population to double in size   

In an environment with unlimited natural resources, population size grows exponentially. One characteristic feature of exponential growth is the time a population takes to double in size.

Exponential growth tends to start slowly, sneaking up before ballooning in just a few doublings.

Real population growth

For real populations, doubling time is not constant. Humans reached 1 billion around 1800, a doubling time of about 300 years; 2 billion in 1927, a doubling time of 127 years; and 4 billion in 1974, a doubling time of 47 years.

On the other hand, world numbers are projected to reach 8 billion around 2023, a doubling time of 49 years, and barring the unforeseen, expected to level off around 10 to 12 billion by 2100.

Earth’s carrying capacity will cut short population growth when premature death by starvation and disease balances the birth rate

This anticipated leveling off signals a harsh biological reality: Human population is being curtailed by the Earth’s carrying capacity, the population at which premature death by starvation and disease balances the birth rate.

Ecological implications

Humans are consuming and polluting natural resources amassed over tens of thousands of years

Humans are consuming and polluting resources – aquifers and ice caps, fertile soil, forests, fisheries and oceans – accumulated over geological time, tens of thousands of years or longer.

The rich consume way out of proportion to their population size

Wealthy countries consume out of proportion to their populations. As a fiscal analogy, we live as if our savings account balance were steady income.

The average American uses about 9.7 hectares of land, compared to only 1.9 hectares used by every person living elsewhere on Earth

According to the Worldwatch Institute, an environmental think tank, the Earth has 1.9 hectares of land per person for growing food and textiles for clothing, supplying wood and absorbing waste. The average American uses about 9.7 hectares.

Bottom line – if every person on Earth were to live like Americans, our planet could only support about 1.5 billion of the current 7.5 billion people

These data alone suggest the Earth can support at most one-fifth of the present population, 1.5 billion people, at an American standard of living.

In 2010, Americans used 1,000 gallons of water per person per day

Water is vital. Biologically, an adult human needs less than 1 gallon of water daily. In 2010, the U.S. used 355 billion gallons of freshwater, over 1,000 gallons (4,000 liters) per person per day. Half was used to generate electricity, one-third for irrigation, and roughly one-tenth for household use: flushing toilets, washing clothes and dishes, and watering lawns.

If 7.5 billion people consumed water at American levels, world usage would be about 11% of the Earth’s total water supply in one year  

If 7.5 billion people consumed water at American levels, world usage would top 10,000 cubic kilometers per year. Total world supply – freshwater lakes and rivers – is about 91,000 cubic kilometers.

Moreover, 2.1 billion people already lack access to safe drinking water

World Health Organization figures show 2.1 billion people lack ready access to safe drinking water, and 4.5 billion lack managed sanitation. Even in industrialized countries, water sources can be contaminated with pathogens, fertilizer and insecticide runoff, heavy metals and fracking effluent.

Freedom to choose

Consumption is rapidly drawing down non-renewable resources including fertile topsoil, drinkable water, forests, fisheries and petroleum.

Though the detailed future of the human species is impossible to predict, basic facts are certain. Water and food are immediate human necessities. Doubling food production would defer the problems of present-day birth rates by at most a few decades. The Earth supports industrialized standards of living only because we are drawing down the “savings account” of non-renewable resources, including fertile topsoil, drinkable water, forests, fisheries and petroleum.

What will happen if present-day birth rates continue?

The drive to reproduce is among the strongest desires, both for couples and for societies. How will humans reshape one of our most cherished expectations – “Be fruitful and multiply” – in the span of one generation? What will happen if present-day birth rates continue?

Population stays constant when couples have about two children who survive to reproductive age. In some parts of the developing world today, couples average three to six children.

We cannot wish natural resources into existence. Couples, however, have the freedom to choose how many children to have. Improvements in women’s rights, education and self-determination generally lead to lower birth rates.

Humans are living far beyond Earth’s sustainable means — Encouraging smaller families, is our most humane hope

As a mathematician, I believe reducing birth rates substantially is our best prospect for raising global standards of living. As a citizen, I believe nudging human behavior, by encouraging smaller families, is our most humane hope.

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